Variable speed gearing for bicycles and the like, generally known as derailleurs, have been known for a number of years. Such variable speed gear systems include two or more main sprockets of different diameters typically fixed to the rear wheel hub. A derailleur arm supports a guide sprocket or pulley for transferring the driving chain from one of the main sprockets to another main sprocket to accomplish a gear change. The derailleur mechanism guides the chain as it is moved from one rear sprocket to another, keeping tension on the chain to prevent a derailment, and also serves to take up and play out the chain as required to accommodate the different main sprocket diameters.
Some have proposed alternative arrangements of derailleur mechanisms to assist with accommodating large amounts of chain, minimizing chain misalignment, providing uniform chain tension or increasing ground clearance. For example, U.S. Pat. No. 4,758,205 issued to Durham describes a pantograph-type guide pulley mechanism in which a floating take-up pulley is linked to a resilient fiberglass rod which is in turn secured to the bicycle frame.
However, such arrangements are not entirely satisfactory. The floating take-up pulley and resilient rod extend well beyond the center of the wheel, even well beyond the outermost main sprocket to connect to the bicycle frame. They thus typically require an intermediate support which extends far outward from the area which bicycle enthusiasts normally consider to be an area free from danger of entanglement with clothing, legs and objects being carried by the bicyclist such as rear mounted bicycle carriers, child seats, and the like. This also causes unwanted vibrations and lateral movement of the sprockets which in turn results in chain misalignment.